Recently, it has become a problem that particulates contained in the exhaust gas discharged from the internal-combustion engine in vehicles such as buses, trucks and the like, or construction machines have caused much harm to environment and the human body.
There are proposed various filters wherein the exhaust gas is passed through a porous ceramic to catch particulates in the exhaust gas and purify the exhaust gas.
As such a ceramic filter, there has been known a honeycomb filter constituted that a plurality of through-holes are arranged side by side in one direction and partition walls separating the through-holes from each other act as a filter.
That is, one end portions of the through-holes formed in the honeycomb filter located at an inlet side or an outlet side of the exhaust gas are plugged with a plugging material so as to form a so-called checkered pattern, wherein the exhaust gas flowed into one through-holes is inevitably passed through the partition walls separating these through-holes and thereafter flowed out from the other through-holes, and as a result, the particulates are caught by the portions of the partition walls in the passing through the partition walls to purify the exhaust gas.
With such a purification of the exhaust gas, the particulates are gradually accumulated at the portions of the partition walls separating the through-holes of the honeycomb filter to cause the clogging to thereby obstruct ventilation. In the above honeycomb filter, therefore, it is required to conduct regeneration treatment by regularly burning and removing the particulates as a cause of clogging with the use of a heating means such as a heater or the like.
In the past, as the honeycomb filter have been known ones consisting of silicon carbide or cordierite, but as the honeycomb filter is heated to a high temperature with the high-temperature exhaust gas in the catching of the particulates or by the heating means such as heater or the like in the regeneration treatment, a filter made of silicon carbide having more excellent heat resistance is considered to be useful.
As the honeycomb filter made of silicon carbide, JP-A-6-182228 discloses, for example, a honeycomb filter produced by using silicon carbide powder as a raw material, shaping it into a predetermined form, drying and thereafter firing (recrystallization method).
And also, JP-A-2001-199777 discloses, for example, a honeycomb structural body obtained by bonding ceramic particles of silicon carbide or the like with a vitreous oxide selected from the group consisting of SiO2, Al2O3, B2O3 and Na2O.
Further, JP-A-2002-60279 and JP-A-2002-154876 disclose, for example, a honeycomb structural body made by adding metallic silicon and an organic binder to silicon carbide powder, mixing and kneading them, shaping into a honeycomb form, and thereafter firing to bond the above silicon carbide powder through metallic silicon.
The honeycomb filter or honeycomb structural body disclosed in the above literatures is set within a cylindrical casing at a state of winding a holding sealing material for performing a role of shock absorption, thermal insulation or the like around the outer periphery while applying a significant compression load, and this casing is arranged in an exhaust passage of an internal-combustion engine to constitute an exhaust gas purification apparatus. By functioning this exhaust gas purification apparatus is conducted the aforementioned purification of the exhaust gas.
Recently, it has been developed to carry a catalyst for removing CO, HC and NOx in the exhaust gas through oxidation and reduction or a catalyst for promoting combustion of caught particulates on the wall portion of the honeycomb filter or the honeycomb structural body disclosed in the above literatures.
Generally, the above catalyst is carried on the wall portion of the honeycomb filter or honeycomb structural body through a catalyst support layer having a large specific surface area such as γ-alumina or the like, but in order to enhance the efficiency of catalytic reaction between the exhaust gas and the catalyst, it is desirable to carry the catalyst as large as possible. For this end, it is necessary to add a large amount of a catalyst support layer to the wall portions of the honeycomb filter or honeycomb structural body as far as possible, whereby it is intended to highly increase the porosity of the honeycomb filter or honeycomb structural body.
However, the honeycomb filter disclosed in JP-A-6-182228 has a problem of becoming brittle and lowering compression strength if the porosity is raised.
Therefore, the above honeycomb filter has a tendency that it becomes easily breakable by compression load added in case of setting within a casing through the above-described holding sealing material, and the setting within the casing becomes difficult.
Furthermore, the above honeycomb filter is used by successively repeating the treatments such as catching and regeneration, so that it is subjected to thermal stress, and there is a problem that if such a thermal stress is applied, the ceramic particles themselves are broken between the ceramic particles to cause visualable size cracks.
And also, the honeycomb structural body disclosed in JP-A-2002-60279 has a structure of bonding silicon carbide particles with the use of metal silicon, so that internal stress can be somewhat mitigated at the bonding point of silicon carbide (metal silicon). Therefore, in case of making the porosity of the honeycomb structural body high, the compression strength is somewhat improved as compared with the honeycomb filter disclosed in JP-A-6-182228, but no improvement by a large margin can be recognized, and there is a problem of breaking by compression load added when setting within the casing.
Since the above honeycomb filter is also used by successively repeating the treatments such as catching and regeneration, so that it is subjected to thermal stress, and there is a problem that if such a thermal stress is applied, fine cracks hardly confirmed visually but confirmable by the observation through SEM are caused in silicon at the bonding portion between the ceramic particles, and as such fine cracks grow, the ceramic particles are broken between the ceramic particles to cause visualable size cracks or removal of the particles.
Further, the honeycomb structural body disclosed in JP-A-2001-199777 has a structure of bonding ceramic particles such as silicon carbide or the like through a vitreous substance and can mitigate internal stress at the bonding point of the ceramic particles (vitreous oxide) as compared with the honeycomb structural bodies disclosed in JP-A-2002-60278 and JP-A-2002-154876, so as to further improve compression strength. However, since the above vitreous oxide changes to be crystalline at a low temperature as compared with metal silicon, there is a problem that when the honeycomb structural body is heated to a high temperature in the regeneration treatment or the like, the vitreous oxide is crystallized to lower the strength of the honeycomb structural body itself, and the structural body is broken by a pressure of the exhaust gas flowed therein.
The present invention is made for solving the above problems inherent to the conventional techniques, and its object is to provide a honeycomb structural body having an excellent compression strength even in the increase of the porosity and being less in the lowering of mechanical strength even in the heating to the high temperature.